I have custom search plugins for work stuff that are absolute lifesavers when combined with the following addons:

Context Search (adds a context menu dropdown when you right click highlighted text; think plaintext words you search in trouble ticket type stuff) https://addons.mozilla.org/en-US/firefox/addon/context-searc...

Add To Search Bar (adds an option to search-engine-ify any submit text box) https://addons.mozilla.org/en-US/firefox/addon/add-to-search... (might as well mention https://addons.mozilla.org/en-US/firefox/addon/organize-sear...)

Bonus productivity favorite: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

So yeah, you'll pry Firefox away from my cold dead hands.